Modern-day armor plate made of specially alloyed and hardened steel is used in military and civilian armored vehicles. While having considerable ballistic resistance and a high armor rating, such plating is still relatively light.
The disadvantage of this material in that it is very difficult to machine and is quite brittle. What is more it has little bending resistance and can develop cracks readily when drilled, welded, or cut.
Thus in order to secure a part, for instance a track idler wheel, to such armor plating, it is standard to provide the part with a flange that is both bolted and welded in place. Bolts pass completely through the flange and armor plate and are secured in nuts welded to the inside surface of the armor plate. In addition the flange itself is welded to the outer surface of the plate.
Such construction has the considerable disadvantage that the welding of the bolts and of the flange produces internal stresses and even tiny cracks in the region where the part is actually supported on the hardened plate. The location of these cracks just where the bending stress is greatest from the attachment can lead to complete joint failure. Repairing such damage is extremely difficult and time-consuming.